Understand compressor types to make the right selection

Compressed air is often called the "fourth utility" when applied to industrial operations. However, unlike water, gas and electricity the consumer is also typically the producer. Becoming your own compressed air utility provider is as simple as buying an air compressor and installing the air lines and associated support equipment.

By John Bartos, Cooper Compressors

01/01/2006

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Compressed air is often called the "fourth utility" when applied to industrial operations. However, unlike water, gas and electricity the consumer is also typically the producer. Becoming your own compressed air utility provider is as simple as buying an air compressor and installing the air lines and associated support equipment.

One of the most important parts of producing compressed air is selecting the appropriate type of compressor for the application. Operators have essentially three types of equipment to choose from in fulfilling their compressed air needs.

The three basic types of air compressors are:

Reciprocating

Rotary Screw

Centrifugal

These types can be further divided into:

oil flooded or oil free

water cooled or air cooled

single stage or multistage

In order to make the best selection it is necessary to develop a fundamental understanding of the principles of operation of each of the three kinds of compressors. While they all perform essentially the same function - taking a volume of air and compressing it from one pressure to a higher pressure, their method of doing so varies significantly. It is this variation in operating principle that makes each type of machine more or less suited to specific types of operation. This article will explain the principles behind each type of compressor and provide an in-depth look at one of the least understood designs - the centrifugal compressor.

Reciprocating Compressors

For most people the principles of operation behind a reciprocating compressor are the easiest to understand. Also commonly referred to as a positive displacement compressor a reciprocating compressor takes in a volume of air, and via a piston connected to a crankshaft forces that initial volume of air into a smaller volume (Figure 1). Taken on its simplest terms the governing physics behind this operation is:

P1 x V1 = P2 x V2

Where the subscript (1) refers to the intake condition and the subscript (2) refers to the discharge. The flow capacity of the compressor is then dictated by the size of the cylinder volume, and the pressure ratio is controlled by the piston stroke.

Reciprocating air compressors cover a broad range of output capacity. They are typically found in applications from 1 horsepower to more than 600 horsepower. The practical limit for a single stage of reciprocating compressors is typically considered to be 5 to 6 ratios. That is, if intake air is considered to be about 15 psi(a) the discharge limit from a single stage is about 85 psi(a). To achieve higher discharge pressures the process is simply repeated in a second compressor piped in series with the first to achieve two-stages of compression.

One advantage of a reciprocating compressor arises from its push-pull action. Compression can be configured to take place on one or both sides of the piston. If only one side of the piston is performing the compression, the process is referred to as single acting. If both sides of the piston are used, the process is referred to as double acting.

To ensure that the highest efficiency of compression is achieved an effective seal between the sliding piston and stationary cylinder is required. Although there are commercially available oil-free reciprocating compressors, it is more typical to see lubricated (also referred to as oil-flooded) machines. Introducing lubricating oil into the cylinder reduces the wear between the piston and cylinder wall but has the undesirable effect of mixing and carrying over into the compressed air stream. As a result, if the process using the air is not tolerant of the presence of lubricant in the air, a downstream separator is required to remove the oil from the air stream.

Typically, oil separators are sized and selected based upon the end use of the air. That end use dictates the oil removal rate and efficiency. For example, if the gas is being used for food or pharmaceutical purposes an additional membrane filter may be required to pass health codes. The primary benefit of reciprocating compressors is their simplicity and initial low cost. The drawback is the frequency of piston and cylinder maintenance and the work involved to replace these parts. From this perspective compressor availability must be considered. In the case of oil lubricated machines, the significant drawback of the additional cost for oil separation systems and maintenance of those systems to produce air quality acceptable for the required process must be calculated into cost of ownership. Due to the relatively harsh nature of their inherent operating characteristics reciprocating compressors often require more substantial foundations than the other two types of equipment.

Rotary Screw Compressors

Although not as obvious as reciprocating compressors, rotary screw compressors are also positive displacement compressors. Compression is achieved via the meshing of two helically cut rotor profiles. One rotor is cut as a male profile, and the other as a female profile. These two rotors spin in opposite direction. For the oil flooded configuration the male rotor drives the female rotor and for the oil free configuration the rotors are kept in precise synchronization via a timing gear. An end view of this arrangement is shown below in Figure 2.

Through an intake port a charge of air is trapped between the meshing lobes. This trapped volume is squeezed along the axis of the rotor into a decreased volume until it reaches the exit port where it is discharged.

The progression of a single pressure pulse through a set of screw rotors is illustrated in figure 3. Here the volume is dictated by the physical size of the rotors as well as the depth of the cut in the pockets. The ratio of compression is governed by the length of the rotor which sets the amount of compression that takes place.

Rotary screw compressors are typically seen in applications ranging from 30 to 350 horsepower. Typically used in plant air applications for 125 to 150 psi(g) the practical discharge pressure limit for rotary screw air compressors is considered to be 250 psi(g).

Although usually configured as single stage machines, rotary screw compressors may also be configured for multiple stages of compression. This can be accomplished with multiple screws within a single body, but can also be carried out in two separate bodies.

Similar to reciprocating compressors, if oil contamination is not desirable or permissible in the process, a downstream separator is required following the compressor. The same care described above for reciprocating compressors and the removal of contaminating lubricant from the process air is applicable to oil flooded screw compressors.

Oil free rotary screw compressors are available in the market. They accomplish the sealing between the rotor bodies and outer casing by means of precision machined sealing strips or abradable coating on both the male and female rotors. The wear characteristics of these sealing mechanisms determine the rate of performance deterioration that the compressor will experience over time.

The primary benefit of rotary screw air compressors is the initial low cost. The drawback is the frequency of maintenance which often entails the complete replacement or overhaul of the rotors or complete air-end. Also, in the case of oil lubricated machines the penalty cost of equipment, maintenance and power for the removal of oil from the compressed air stream can also be viewed as detrimental.

Centrifugal Compressors

While centrifugal compressors accomplish the same results as the previously described compressor types, they go about it in an entirely different way. Whereas reciprocating and screw compressors compress air by squeezing the air from a large volume into a smaller one, centrifugal compressors raise pressure by increasing the air's velocity. For this reason, centrifugal compressors are referred to as dynamic compressors.

Centrifugal compressors raise the pressure of air by imparting velocity, using a rotating impeller, and converting it to pressure. Each stage of compression in a centrifugal compressor consists of an impeller which rotates and a stationary inlet and discharge section. Air is directed into the "eye" of the spinning impeller through the inlet. The impeller imparts velocity to the air and discharges it into the diffuser where the velocity is converted to pressure. Figure 4 shows the typical components of a centrifugal compressor stage.

Each stage of compression can achieve in practical terms approximately 2.2 up to 3.0 ratios in air service. Due to the heat of compression interstage cooling is required and can be accomplished either via air-to-air or more commonly water-to-air cooling.

Centrifugal compressors are oil free. The air flow path and the oil system are independent, separated by seals and an atmospheric air space. Any lubricant required for the bearings or other mechanical components is sealed off from the air stream. This feature has the benefit of not requiring any downstream separation. Centrifugal compressors are ideally suited to processes where uncontaminated air is required for the application.

Centrifugal compressors are recognized for their ability to operate for long periods without required maintenance. They have a minimum number of moving parts, and do not rely on contact between parts to accomplish the compression process. A unique advantage of centrifugal compressors due to their dynamic nature is the lack of pressure pulsations. This makes them inherently quieter than their positive displacement counterparts and also results in smoother operation.

Overall, centrifugal compressors run at very low vibration levels which minimize the foundation requirements and associated piping supports. Pulsation bottles or receiver tanks are not required downstream of centrifugal compressors as is the case with positive displacement compressors. The result is that the installation cost for centrifugal compressors in lower than positive displacement compressors.

The Bottom Line...

It is important to evaluate your specific application to determine the best type of compressor to be used.

The most accurate method is to calculate the life cycle cost of the machine. In the calculation the key factors will be equipment cost, operating efficiency (power consumption), installation cost, and maintenance cost.

If an oil-flooded compressor is being considered, the oil removal system and the cost of maintaining the proper filtration must be included.

For a realistic comparison, a total life of 10 years would be a minimum.

Fig. 1. Reciprocating compressor principal of operation is based on the relationship P1 x V1 = P2 x V2, assuming constant temperature. The subscript 1 refers to the intake condition, and the subscript 2 refers to the discharge. The flow capacity of the compress or is then dictated by the size of the cylinder volume, and the pressure ratio is controlled by the piston stroke.

Fig. 2. End view of intermeshing lobes in a screw compressor illustrates how air is trapped in the cavity.

Fig. 3. Compression occurs in a screw compressor as entrapped air travels from the intake to the discharge.

Fig. 4. A single stage of a centrifugal compressor comprises a rotating impeller in a stationary shroud with an inlet and discharge sections. Air is compressed by increasing its velocity within a fixed volume.

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